(1) Field of the Invention
The present invention relates to the field of data communications. More particularly the present invention relates to the field of efficient signal transmissions over high speed communication links.
(2) Prior Art
Small differential signals provide a number of advantages for the transmission of high bit rate data signals. A differential channel provides rejection of common mode noise (such as power supply noise) that may be present between a transmitting ("driving") and a receiving node. Differential signals can be transmitted on well known twisted-pair cables which are less expensive than coaxial or fiber optic cables and which when shielded, offer very good rejection of interferences from sources of external noise. Using small signal level differential signals over a shielded twisted-pair cable reduces EMI emissions, simplifies transmitter design, and reduces power dissipation.
One characteristic of a differential communication channel is that it reduces timing distortion due to mismatched rise and fall times and receiver threshold. Timing distortion must be minimized since in a digital communication system data is encoded in both time and amplitude. It is very difficult in a single-ended communication system to match the rise and fall times and this mismatch becomes significant when data rates become high (e.g., 50 Mbaud or more). Differential systems, however, do not suffer duty cycle distortion due to rise and fall time mismatch. Also, within a differential receiver, the threshold is not set externally as in a single-ended system. Instead, the threshold in a differential system is a function of the received signal and therefore tracks with the received signal corresponding to when the differential signal equals zero volts. As such, it would be advantageous to provide high speed communication using a differential signal configuration.
Twisted pair drivers may utilize a bias voltage applied between terminating resistors of a twisted pair cable. For a given twisted pair cable, the bias voltage of the driver circuit that couples two devices is in some instances generated by a single device. As such, with respect to a single twisted pair cable, two circuits may utilize or share the bias voltage generated by one of the devices. Therefore, depending on the coupled device to an overall communication network, the bias voltage may vary somewhat from device to device. For instance, an exemplary bias voltage generated by one device (ideal) may be 1.8 volts, but the voltage received by the other device may range widely from 1.0 volt to 2.8 volts for a typical range of devices coupled to a communication network. It would be advantageous to provide a communication apparatus that operates within this wide range of bias voltages and the present invention provides such functionality.
A wide received bias voltage (common mode voltage) range may also occur if one device has a lower or higher ground node than another coupled device. In this case the bias voltage perceived by one device may be slightly higher or lower than the bias voltage generated by the other device (e.g., by about +0.5 to 1 volt). If one device receives a high or low external bias voltage because of the variations in ground node voltages, then this will typically cause an imbalance in the driver circuit and communication errors may occur.
The wide common mode voltage range as described above, depending on the driver circuit design, may cause common mode current flow into or out of the twisted pair driver. This is unwanted since the additional current supplied by the driver may be detected as a common mode signal across the twisted pair cable and this type of common mode signal may decrease the margin for speed signaling detection which utilizes a particular signal characteristic under the IEEE P1394 communication standard. An undesirable event may occur wherein the common mode current from the driver may appear improperly as a recognized common mode speed signal under the above IEEE standard and may be improperly perceived as proper speed information. As such, it is advantageous to provide a driver circuit that does not generate common mode current signals as a result of a wide common mode voltage range arrangements between coupled devices. The present invention offers such advantageous functionality.
One solution to the problem of unwanted common mode current flow resulting from above variations in the common mode bias voltage is to utilize p-type and n-type transistors having larger channel lengths to act as the driving transistors over the twisted pair cable. By increasing the channel length dimensions the transistors will have increased output impedance. This larger output impedance tends to reduce unwanted common mode current over the twisted pair cable. However, it is not advantageous to utilize larger p-channel transistors because larger channel length transistors are slow and decrease the overall communication rate of the driver and also increase the die size required of the integrated circuit on which the drivers are implemented. It is advantageous, for high speed communication, to use relatively small channel length transistors. The present invention provides a driver circuit that does not allow significant common mode current in response to wide common mode bias voltage range as described above yet utilizes relatively small channel length transistors.
Accordingly, it is an object of the present invention to provide efficient communication between coupled devices. It is another object of the present invention to provide a twisted pair driver circuit that does not generate significant common mode current in response to a wide common mode range. It is further an object of the present invention to provide a driver circuit that restricts improper common mode current flow when devices having different ground node voltages (e.g., ground drops) are coupled together. It is an object of the present invention to provide a driver circuit that may be compatible with a variety of communication devices having a wide range of bias voltages. It is yet another object of the present invention to provide a driver circuit that may operate with a common mode bias voltage source having a reduced current supply capacity. These and other objects not specifically mentioned above will become clear within discussions of the present invention to follow.